[Haskell-cafe] Clarifying a mis-understanding about regions (and iteratees)

Yves Parès yves.pares at gmail.com
Thu Feb 23 09:37:16 CET 2012

Hi, so there are different regions libraries?
Is there a shootout comparing them, possibly also with ResourceT from
conduit (which has also been implemented in a stand-alone package
http://hackage.haskell.org/package/resource-simple-0.1 by Robin Banks), for
I take it it tries to respond to the same problem?

2012/2/23 <oleg at okmij.org>

> I have just come across the reddit discussion about regions and
> iteratees.
> http://www.reddit.com/r/haskell/comments/orh4f/combining_regions_and_iteratees/
> There is an unfortunate misunderstanding about regions which I'd like to
> correct. I'm posting in Cafe in hope that some of the participants of
> that discussion read Haskell-Cafe (I'm not a redditor).
> The user Faucelme wrote:
> > Would it be possible to implement a region-based iteratee which opened
> some
> > file "A" and wrote to it, then opened some file "B" and wrote to it,
> while
> > ensuring that file A is closed before file B is opened?
> To which the user tibbe replied
> > You can typically explicitly close the files as well.
> and the user dobryak commented
> > Regions that Oleg refers to started out with region-based memory
> allocation,
> > which is effectively a stack allocation strategy, in which resource
> > deallocation is in LIFO order. So I think your assumption is correct.
> Regretfully, these comments are incorrect. First of all, memory
> regions were invented to get around the stack allocation, LIFO
> strategy. If the stack allocation sufficed, why do we need heap?  We
> have heap specifically because the memory allocation patterns are
> complex: a function may allocate memory that outlives it.  Regions
> let the programmer create arbitrarily many nested regions; everything
> in a parent region is available to a child. Crucially, a child may
> request any of its ancestors to allocate memory in their
> regions. Therefore, although regions are nested, memory does not have
> to be allocated and freed in LIFO order.
> The Lightweight monadic regions implement all these patterns for files
> or other resources (plus the dynamic bequest).
>        http://okmij.org/ftp/Computation/resource-aware-prog/region-io.pdf
> The running example of the Haskell Symposium 08 paper was the
> following (see sec 1.1)
> 1. open two files for reading, one of them a configuration file;
> 2. read the name of an output file (such as the log file) from the
>   configuration file;
> 3. open the output file and zip the contents of both input files into
>   the output file;
> 4. close the configuration file;
> 5. copy the rest, if any, of the other input file to the output file.
> As you can see, the pattern of opening and closing is non-LIFO: the
> output file has to be opened after the configuration file and is
> closed also after the configuration file. Therefore, the user Faucelme
> can find the solution to his question in the code accompanying the
> Monadic Regions paper.
> Section 5 of the paper describes even more complex example:
> 1. open a configuration file;
> 2. read the names of two log files from the configuration file;
> 3. open the two log files and read a dated entry from each;
> 4. close the configuration file and the newer log file;
> 5. continue processing the older log file;
> 6. close the older log file.
> where the pattern of opening and closing is not statically known:
> it is decided on values read from the files.
> So, Faucelme's question can be answered in affirmative using the
> existing RegionIO library (which, as has been shown, well integrates
> with Iteratees). There is already a region library with the desired
> functionality.
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